1. Field of the Invention
The present invention relates to a process for depositing an at least partially crystalline silicon layer on a substrate using a plasma, wherein a plasma is generated and the substrate is exposed to a silicon-containing source fluid for deposition of silicon therefrom. The invention also relates to a device for depositing an at least partially crystalline semiconductor layer on a substrate, comprising a plasma chamber for generating a plasma therein, a reaction chamber provided with a substrate holder which is in open communication with the plasma chamber via a passage opening, and supply means for a source fluid.
Such a process and device find particular application in the modern semiconductor industry and solar cell industry, and then mainly for the manufacture of thin-film solar cells and thin-film transistors (TFT) and diodes, these latter being applied particularly for driving of image reproducing systems on the basis of liquid crystal cells and other solid-state elements. These semiconductor elements are generally formed in a thin silicon layer, wherein the silicon layer itself lies on an underlying substrate. In view of the explosive growth in the demand for such image reproducing systems and the expectation that the demand for solar cells as alternative energy source will also greatly increase in the near future, a process of the type stated in the preamble, with which a high-quality silicon layer can be formed in economic manner, is of exceptionally great importance.
2. Description of the Related Art
Different processes of forming a silicon layer on an underlying substrate are per se known. These processes consist on the one hand of deposition techniques wherein a silicon layer can be deposited from a chemical vapour of a silicon-containing source fluid. Such processes are designated CVD (Chemical Vapour Deposition), frequently with the addition of “RF Plasma Enhanced” or “Hot Wire” to indicate that a radio-frequency plasma or a hot wire, usually of tungsten, is used to support the process. The morphology of the deposited layer herein varies from strictly amorphous to microcrystalline and polycrystalline. In addition, there are epitaxial growing techniques wherein a silicon layer is thickened epitaxially after a seed layer has first been formed using for instance a laser technology.
A drawback to these known techniques is however the relatively low growth speed of often no more than only a few tenths of a nanometre per second and, in the case of epitaxial growth, the relatively high temperature to which the substrate has to be exposed to obtain a high-quality crystalline silicon layer. These techniques are therefore less suitable for large-scale industrial application, and significant demands are also made in respect of the heat resistance of the substrate. For this reason these known methods particularly do not lend themselves for a new development in the field of solar cells, wherein these are applied in a silicon layer of typically 1 μm thickness on a foil of a plastic usually having little heat resistance. The efficiency of these existing techniques, in which a usually relatively expensive source fluid is used, is moreover comparatively low where they are used.